Articles having a fine concave-convex structure with a pitch less than or equal to the wavelength of visible light on a surface thereof, express an anti-reflective function or the like, and thus the usefulness thereof is currently attracting attention. Particularly, a fine concave-convex structure called a Moth-Eye structure has been known to express an effective anti-reflective function due to a continuous increase from the refractive index of the air to the refractive index of a material.
As methods of manufacturing an article having a fine concave-convex structure on a surface thereof, the following methods, (i) or (ii), are known, and the method of (ii) is excellent from the viewpoint of productivity and economic efficiency.
(i) A method of manufacturing an article having a fine concave-convex structure on the surface thereof by directly processing a surface such as a transparent substrate.
(ii) A method of transferring an inversion structure onto a surface such as a transparent substrate using a mold having an inversion structure corresponding to a fine concave-convex structure.
As a method of forming an inversion structure in a mold, an electron beam lithography method, a laser light interference method and the like are known. In recent years, as a method in which an inversion structure is capable of being formed more easily, a method of anodizing a surface of an aluminum substrate has attracted attention (see, for example, PTL 1).
Anodized alumina formed by anodizing the surface of the aluminum substrate is an aluminum oxide film (alumite), and has a fine concave-convex structure composed of two or more pores in which the pitch is less than or equal to the wavelength of visible light.
In the above-mentioned anodized alumina, when non-uniformity occurs in the concentration or the temperature of an electrolyte for immersing the aluminum substrate at the time of anodization, or non-uniformity occurs in the surface texture of the aluminum substrate, a slight non-uniformity may occur in the depth of the pores and the pitch between the pores. When the area of a region in which the depth of the pores of the anodized alumina and the pitch between the pores are not formed as designed is increased, such a mold is not able to be used in manufacturing an article having a fine concave-convex structure on the surface thereof. However, a method in which the depth of the pore of anodized alumina and the pitch between the pores can be easily inspected has not been available.
In addition, a mold release agent is attached to the surface of a mold including an anodized alumina layer having such a fine concave-convex structure in order to enhance the mold releasability of a molded article. When an excessive amount of mold release agent is attached to the surface of the anodized alumina layer having a fine concave-convex structure, the fine concave-convex structure is buried by the mold release agent, and thus a problem occurs in that an optimum fine concave-convex structure is not transferred onto the surface such as a transparent substrate. On the other hand, when the amount of the mold release agent is extremely small, desired mold releasability is not obtained, and thus a problem occurs. For this reason, it is necessary to perceive the attachment state of the mold release agent on the surface of the anodized alumina layer having a fine concave-convex structure, but there is a problem in that currently, there is not a method for easily inspecting the attachment state of the mold release agent on the surface of anodized alumina.
In addition, although such a mold, which makes use of the anodized alumina, is manufactured by anodizing the aluminum substrate, a material used in the aluminum substrate serving as a material of the anodized alumina is of a high purity. However, the crystal grains of high-purity aluminum used for manufacturing the high-purity aluminum substrate have a tendency to be coarsened due to casting or the like, and coarse crystal grains that can even be seen with the naked eye are generated in the aluminum substrate. For this reason, there is a problem in that the same grain boundary pattern is generated even in the surface of the anodized alumina of the mold manufactured using such an aluminum substrate, and such a grain boundary pattern is even transferred onto the surface of an article such as an optical sheet manufactured using this mold.
Consequently, in order to make a pattern caused by the crystal grains refined and uniform, the aluminum substrate is manufactured by rolling, extruding, forging, or the like. However, even when such processes are performed, the traces of the coarse crystal grains at the time of casting before forging cause non-uniformity in the crystal orientation depending on the conditions of the processing direction, the number of repetitions or the like, and thus remain in the aluminum substrate. The cicatrix thereof may remain in the mold having a fine concave-convex structure processed from the aluminum substrate, as a flow pattern which is difficult to visually recognize with the naked eye. This flow pattern is difficult to visually recognize on the mold. However, when optical sheet and the like are manufactured using the mold, the flow pattern is transferred onto the optical sheet as a flow pattern (in which a mark of a liquid flow remains) such as marbling on the sheet surface which is visible to the naked eye, which results in an appearance defect.
The optical sheet onto which the fine concave-convex structure of anodized porous alumina is transferred has excellent anti-reflection performance. Therefore, when the optical sheet is attached to the surface of an electronic display device and a dark image is displayed, the difference in reflectance or transmittance of slight light tends to be conspicuously visually recognized as a defect. For this reason, non-uniformity of the surface of the aluminum substrate in the crystal orientation which is not visually recognized on the mold is transferred onto the optical sheet as a defect and tends to be conspicuously visually recognized.
Further, there is a problem in that there is no method capable of easily inspecting the flow pattern of the mold having an anodized alumina layer formed by anodization on the surface thereof.